This invention concerns a porous structure for abstracting dissolved metal ions from aqueous solutions, and further concerns a method for gold recovery utilizing said structure.
The use of ion exchange resins for the absorption of dissolved ions is well known. Such resins are generally manufactured in bead form and are utilized in columns through which liquids are passed to interact with the beads. The beads are generally made of cross-linked polystyrene which is then chemically modified. Two chemical modifications of such resins are prevalent, namely: a resin having sulfonic acid groups and capable of exchanging positively charged ions (cations), and a resin having quaternary ammonium groups and capable of exchanging negatively charged ions (anions). Except for their selectivity for either cations or anions, such resins do not generally distinguish between different kinds of cations or anions as may be found in multi-component solute mixtures.
A type of ion exchange resin capable of selectively removing certain ions from mixtures has been available as a chelating resin wherein the cross-linked polystyrene beads are caused to contain imino diacetic acid radicals of the formula: EQU -N(CH.sub.2 --CO.sub.2 H).sub.2
For example, a bed of such chelating resin is capable of abstracting Cu++ from a solution containing Cu++ and Na+. Such chelating resins, however, are limited to the absorption of metal cations capable of forming a ring structure with the carboxyl groups of the iminodiacetic acid radicals.
In the mining of gold by leaching methods, fine particles of gold become dissolved in the leach solution as anions. In particular, when sodium cyanide is employed as the active component of the leach solution, the gold is dissolved as the aurocyanide anion Au(CN).sub.2.sup.- . In current practice, the leach solution containing dissolved gold in the form of the aurocyanide anion is treated with activated carbon in granular form. The carbon absorbs Au(CN).sub.2.sup.- in the presence of the high concentrations of CN.sup.-. However, other metals present in the leach solution as cyanide-containing anions are indiscriminately absorbed, thereby diminishing the capacity of the carbon for gold. Among such other metals that may occur in the leach solution, silver is the most common, but copper, iron, cobalt and other metals are also encountered. The effectiveness of the carbon is nevertheless superior to anion exchange resins which would not distinguish between the large concentration of CN.sup.- and relatively low concentration of Au(CN).sub.2.sup.- .
The granular activated carbon utilized in the recovery of gold from leach solutions must be confined by special retention means which permit contact of the granules with the leach solution, yet retain the granules. The most commonly employed retention means is a cylindrical column which holds the granules as a bed, yet permits passage of the solution undergoing treatment. Such beds of granules are susceptible to clogging by accumulation of deposits of material suspended in the leach solution.
An ion exchange resin attached to an open-celled cellulose sponge is described in U.S. Pat. No. 4,332,916. Because such product is self-supporting and of potentially large size, it can be confined in retention means other than columns, and is substantially unaffected by water-borne suspended matter. In view of its physical characteristics, it is eminently suited for the treatment of flowing natural waters where high flow volumes and high sediment levels are encountered. The ion exchange resin of U.S. Pat. No. 4,332,916 is essentially a cation exchange resin chemically bonded to the hydroxyl groups of the cellulose of the sponge by means of a cross-linking agent. One of such resins is based upon a high molecular weight polyethyleneimine (PEI) interacted with 75% by weight of the crosslinker 2-butenyl-bis [1, 4 - bis (2 hydroxyethyl) sulfonium chloride]. The very high cost of the crosslinker causes the product to be too expensive to be practical.
A chelating polymer produced by the interaction of PEI with nitrilotriacetic acid (NTA) is disclosed in U.S. Pat. No. 3,715,339. By utilizing a high ratio of NTA to PEI, the chelating polymer has a preponderance of pendant carboxyl groups which afford high affinity for multi-valent cations. Such polymers absorb water in the amount of between 100 and 600 percent of the dry weight of the resin to achieve a gel-like consistency. Accordingly, a bed of granules of such polymer presents a very high resistance to the flow of liquid therethrough. Although U.S. Pat. No. 3,715,339 discloses the disposition of the polymer within a permeable matrix such as open-celled sponge, it is implied that the polymer would occupy the cells of the sponge. As in the case of conventional ion exchange resins, absorbed ions can be eluted to permit re-cycling of the resin.
In various pollution control applications, it is sought to remove dissolved metals in the form of anions from naturally occurring flowing streams or industrial effluents. Typical species include: CrO.sub.4.sup.-2 ; UO.sub.2 (CO.sub.3).sub.3.sup.-4 ; AsO.sub.4.sup.-4 ; Se.sub.3.sup.-2 ; SeO.sub.3.sup.-2 ; MoO.sub.4.sup.-2 ; SnO.sub.3.sup.-2. The removal of such species requires a highly selective resin capable of processing a large volume of rapidly flowing water without fouling by suspended matter within the water.
It is accordingly an object of the present invention to provide a highly porous product capable of selectively absorbing anions from aqueous solution.
It is another object of this invention to provide a product as in the foregoing object having high selectivity for gold in a leach solution which contains gold in an anionic form, especially the aurocyanide anion.
It is a further object of the present invention to provide a product of the aforesaid nature capable of re-cycling through absorption and elution cycles.
It is a still further object of this invention to provide a product of the aforesaid nature which provides little impedence to the flow of water therethrough, and is relatively unaffected by the presence of suspended matter in said water.
It is yet another object of the present invention to provide a product of the aforesaid nature in the form of an open-celled sponge of controllable size and amenable to low cost manufacture.
These objects and other objects and advantages of the invention will be apparent from the following description.